Composition for lithographic plate coatings



Patented Sept. 28, 1954 COMPOSITION FOR LITHOGRAPHIO PLATE COATIN GSVernon L. Gregory, Piedmont, Calif., assignor to Gregory & Falk, SanFrancisco, Calif., a co,-

partnership No Drawing. Application November 10, 1950, Serial N0.195,131

4 Claims. (01. 95-7) This invention relates to the offset printingprocess, and is particularly directed to an improved coating forlithographic plates utilized in such a process.

In modern lithography, usually a zinc or aluminum plate is provided witha dried surface coating, and thereafter, the coated plate is exposed tolight through a photographic film or the like whereby the exposedsurface of the coating forms a relatively hard film, while the unexposedsurface remains soft and water soluble. A suitable developing ink isthen spread evenly over the plate and dried. After washing oif the watersoluble portion of the coating, the remaining exposed or hardened film,which is receptive to greasy material, may be covered with alithographic ink, and in the offset process, the inked design on theplate is transferred to a rubber blanket which is then brought intocontact with the paper. With the surface coatings heretofore utilized,up to approximately 100,000 reproductions could be made from a plateunder ideal conditions, but as an average, only 20,000 of suchreproductions could be reasonably anticipated before the copies :becameblurred, indistinct or otherwise objectionable. These previously usedcoatings usually included a colloid solution of albumen, dex'trin,casein or one of the soluble gums rendered light sensitive by theaddition of a material such as ammonium bichromate or the sodium orpotassium salt of bichromate. When a plate provided with such a coatingis exposed to light, the exposed colloid and bichromate forms thehardened film which is receptive to lithographic ink; but as copies arereproduced by the plate in the aforementioned manner, the hardened filmprogressively loses its ink receptivity due in part to the physicalwearing away of the film, and believed also to be due to chemicalchanges occurring therein. In connection with this latter point, it hasalso been found that if a plate is stored for any considerable length oftime, the ink receptivity of the film is lessened, thereby resulting inpoor reproductions.

It is therefore an object of the present invention to provide a coatingfor lithographic plates which will permit a maximum number of sharp,distinct reproductions to be printed thereby.

Another object of the invention is to provide a coating for lithographicplates which will retain its ink receptivity over a longer period oftime and use than the coatings heretofore utilized in the art.

It is a further object of this invention to provide a coating possessingthe above characteristics which will retain its ink receptivity evenafter the exposed surface thereof is worn away by contact with therubber blanket, paper, ink, or other foreign material.

The invention possesses other objects and features of advantage which,together with the foregoing, will be specifically set forth in thefollowing detailed description of the invention. It will be understoodthat the invention is not to be limited to the specific compositions orprocedure herein described, as variations thereof may be employed whichcome within the scope of the appended claims.

In broad terms, the present invention relates to an image formingcoating for lithographic plates, and includes a fatty substanceemulsified with a colloid that is rendered light sensitive by thepresence of ammonium bichroinate. Such a coating is utilized in themanner well known in the art as previously described, and it has beenfound that the addition of the fatty substance to the coating permitsthe latter to retain its ink receptivity over longer periods of time andunder more conditions of use than any of the coatings previously used.This important factor of increased ink receptivity results from thenatural afiinity of ink with fatty substances, and it is also believedthat the metal plate itself is chemically changed by the fattycomposition so that even after substantially all of the coating is wornaway, the plate will still be receptive to the lithographic ink.

Although numerous fatty substances could be utilized in the composition,it is far more desirable to provide a natural fatty emulsion rather thanrequiring the addition of an emulsifier to other types of fat mixtures.For purpose of explanation, I have found the following substances togive excellent results in forming natural fatty emulsions when mixedwith water: dried milk, dried whole egg, or natural soya bean powder. Ifdesired, the emulsion may comprise liquid whole milk or syntheticmixtures of a fat and an emulsifier and water.

In actual practice, the plate coating composition is usually prepared indried powder form, that is, the colloid base, the light sensitivematerial, and the fatty substance are thoroughly physically mixedtogether so as to form a substantially uniform mixture. In this driedcondition, the composition may be easily shipped and stored until readyfor use. When it is desired to utilize the same for a plate coating, asulficient quantity of the mixture is added to the necessary amount ofwater in the familiar manner so as to produce the fatty emulsioncomposition heretofore described. The lithographic plate is then coatedwith the latter composition preparatory to light exposure.

To enable those skilled in the art to practice this invention, I havehereinbelow set forth examples of compositions which may successfully beutilized in forming lithographic plate coatings of the type desired.

Example N0. 1

Mix 800 ounces of a colloid such as casein, dextrin, albumen, or one ofthe soluble gums, with 320 ounces of a light sensitive material such asthe potassium or sodium salts of bichromate, but preferably ammoniumbichromate, and 230 ounces of dry whole milk powder, dried whole egg, ornatural soya bean powder. If desired, approximately 12 ounces of thymolmay be added to the above, the thymol acting as a preservative andodorant for the mixture. Sufficient water may then be added to themixture to make 680 quarts of solution.

Example No. 2

If it is desired to use liquid whole milk, the amounts of colloid, lightsensitive material and thymol will be the same as in the above exampleto which will be added 106 quarts of whole milk. Here too, sufficientwater will be added to make up 680 quarts of solution.

It will be understood that the figures given in the foregoing examplesare merely by way of illustration, as it is possible to deviatetherefrom without departing from the scope of this invention. From theseexampl%, it will be seen that of the total dry weight of the mixture,approximately 60% represents the colloid, 25% represents the bichromateor other light sensi tive material, and 15% represents the dried fattysubstance. However, with regard to this latter percentage, it will benoted that a material such as dried whole milk has approximately a 30%fat content, and therefore, the actual fat content of the dried mixtureis only in the neighborhood of 5% of the total weight. Although this fatcontent may be increased if the user takes extra precautions indeveloping the lithographic plate, I have found that by keeping the fatcontent in the above proportion, there will be little danger ofexcessive fat causing a soft exposed image on the plate.

The foregoing percentages are believed to give optimum results with aminimum of safeguards required to be employed by the user thereof.However, mixtures may and have been more or less successfully utilizedin which the percentages of the ingredients to the total dry weight havefallen within the following ranges:

Per cent Colloid 40 to 80 Bichromate 5 to 50 Fat 0.5 to 20 It will thusbe seen that a degree of flexibility is offered without radicallydeparting or deviating from the present invention.

I claim:

1. An image forming coating for lithographic plates comprising a colloidselected from the group consisting of albumen, casein, and a solublegum; a light sensitive material selected from the group consisting ofammonium bichromate, sodium bichromate, and potassium bichromate; and asubstance which will form an emulsion in water selected from the groupconsisting of dried milk, dried whole egg, and natural soya bean powder.

2. An image forming coating for lithographic plates comprising a colloidselected from the group consisting of albumen, casein, and a solublegum; a light sensitive material selected from the group consisting ofammonium bichromate, sodium bichromate, and potassium bichromate; andliquid whole milk.

3. Am image forming coating composition for lithographic platesincluding a natural saturated fatty substance selected from the groupconsisting of dried milk, dried whole egg and natural soya bean powder,the fat content of said fatty substance constituting from 0.5% to 20% ofthe total dry weight of the composition, a colloid, and a bichromatesalt.

4. A dry composition for use as a lithographic plate coating comprisinga colloid selected from the group consisting of albumen, dextron, caseinand a soluble gum; a light sensitive bichromate salt; and a naturalsaturated fatty substance selected from the group consisting of milk,whole egg and natural soya bean and capable of forming an emulsion inwater; the said colloid constituting approximately 60% by weight of thecomposition, and the fat content of the said fatty substanceconstituting approximately 5% by weight of the composition.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 66,102 Morvan June 25, 1867 134,470 Edwards Dec. 31, 1872209,036 Freeman June 8, 1878 556,380 Stevens Mar. 17, 1896 2,574,258Frost Nov. 6, 1951 2,598,189 Mullen May 27, 1952 2,624,673 Frost Jan. 6,1953 FOREIGN PATENTS Number Country Date 267 Great Britain July 28, 186312,867 Great Britain Feb. 8, 1906 517,686 Great Britain Feb. 6, 1940

1. AN IMAGE FORMING COATING FOR LITHOGRAPHIC PLATES COMPRISING A COLLOIDSELECTED FROM THE GROUP CONSISTING OF ALBUMEN, CASEIN, AND A SOLUBLEGUM; A LIGHT SENSITIVE MATERIAL SELECTED FROM THE GROUP CONSISTING OFAMMONIUM BICHROMATE, SODIUM BICHROMATE, AND POTASSIUM BICHROMATE; AND ASUBSTANCE WHICH WILL FORM AN EMULSION IN WATER SELECTED FROM THE GROUPCONSISTING OF DRIED MILK, DRIED WHOLE EGG, AND NATURAL SOYA BEAN POWDER.